Field of the Invention
The present invention relates to a method for producing a transistor structure, in particular a MOS transistor structure, and to a transistor structure.
In many applications of modern electronics, the problem arises that, besides the pure processing of information, the electronic product must also interact with its environment in some way. One example of this involves the control system for an airbag, which on the one hand evaluates the signals measured by an acceleration sensor and on the other hand triggers the airbag in the event of an accident.
For economic reasons, it would be advantageous if all these different functions could be integrated in a single semiconductor product. However, the production of such "smart-power" products places great demands on the production process which is used. Thus, for example, a variety of component types, such as CMOS transistors, DMOS power transistors and bipolar transistors, need to be integrated with high packing density on a chip. However, the production process should at the same time involve a minimum number of dopant implantation and dopant diffusion steps, and few masking planes.
The number of dopant implantation and dopant diffusion steps is customarily reduced by using one dopant implantation and dopant diffusion step, as well as the resulting dopant profile, for a plurality of different components. In this case, however, it must be taken into account that different components have different requirements in terms of their doping profile. For example NMOS and DMOS transistors require a specific surface concentration of the dopant in the region of the p-channel in order to give a defined transistor threshold voltage. In the case of JFETs and controllable resistors, it is advantageous to provide a weakly doped flat P-area. However, in order to improve the invulnerability of an NMOS transistor to latch-up, it is necessary to form a p-well with a low film resistance. A deep rectangular profile would be ideal for this purpose.
Since these different requirements cannot be covered by one doping profile, but on cost grounds it is only possible to have a few dopant implantation and dopant diffusion steps, it has not to date been possible to set optimum doping profiles for the respective component. In particular, it has not been possible to produce a p-well with a deep rectangular profile, as would be beneficial for NMOS transistors, since this would result in a very high degree of lateral diffusion out of the p-wells, in particular the p-wells of other components.